The present invention relates to an aromatic polyester, more particularly an aromatic polyester for optical applications.
Conventional aromatic polyesters are prepared by the following process. Aromatic polycarboxylic acid halide, e.g., terephthalic or isophthalic acid chloride, is dissolved in an organic solvent, e.g., methylene chloride, while 2,2-bis(4′-hydroxyphenyl)propane[bisphenol A] is dissolved in an aqueous alkali solution, e.g., aqueous sodium hydroxide solution. Then, these solutions are mixed at a room temperature in the presence of a surfactant, e.g., a quaternary ammonium salt to yield aromatic polyester by interfacial polymerization (Japanese Patent Publication No. S55(1980)-4771). The resulting aromatic polyester has high heat resistance, therefore it is useful. In the case where the molar ratio of terephthalic or isophthalic acid chloride to bisphenol A is close to 1, the resulting aromatic polyester is crystalline and insoluble in solvents due to extraordinarily high molecular weight, or has a high melting point of not less than 300° C., even if the polyester is dissolved in solvents. Accordingly, for processing such as injection molding, the polymer must be heated to a temperature of 300° C. or higher in order to enhance the fluidity during the processing, leading to formation of an undesirably colored product which cannot be used in optical applications requiring high transparency.
A disclosed polyester sheet having high transparency and heat resistance is a heat-resistant transparent multilayered plastic sheet including a core layer composed of a thermoplastic polyester having a predetermined number of ethylene terephthalate units, intermediate layers composed of a resin composition comprising a thermoplastic polyester and a polyarylate or a resin comprising a polyarylate only, the intermediate layers being laminated both sides of the core layer, and outermost layers composed of a copolyester prepared by replacing a part of the glycol component of the polyethylene terephthalate with 1,4-cyclohexanedimethanol, each of the layers having a predetermined thickness (Japanese Patent Application Laid-Open No. H02(1990)-276639). This patent application document discloses that copolyesters can be prepared by replacing a part of the glycol component of polyethylene terephthalate with 1,4-cyclohexane dimethanol, which is a concept of multi-component polyester. The concept, however, relates to polyethylene terephthalate only, and its purpose is focused on an improvement in heat resistance and transparency of material for food containers.